var r={
  'special':/[\W]/g,
  'quotes':/['\''&'\"']/g,
  'notnumbers':/[^\d]/g,
  'data':/[^\d/]/g
}

function valid(o,w){
  o.value = o.value.replace(r[w],'');
}

//// utilizacao: echo tep_draw_input_field('postcode','','maxlength=8 onkeyup="valid(this,\'notnumbers\')" onblur="valid(this,\'notnumbers\')"')